The present invention relates to a mast apparatus for fork lift trucks.
More specifically, the invention relates to a mast apparatus as xe2x80x9ca mast with a mechanism for having an unchanged mast height until a lift bracket reaches an upper end of an inner mastxe2x80x9d (hereafter sometimes called xe2x80x9ca full free lift mastxe2x80x9d), as xe2x80x9ca mast of a system having outer mast faces as guides for an inner mast to slide up and downxe2x80x9d (hereafter sometimes called xe2x80x9ca slide type mastxe2x80x9d) and as a mast of a triple mast type having a pair of left and right full free lift cylinders independently provided or separated from each other and a pair of left and right lift cylinders likewise provided for a load handling service, as well as to a mast apparatus as a full free lift mast of a triple mast type.
There is disclosed in Japanese Utility Model Application Laid-Open Publication No. 59-123096, as well as in Japanese Utility Model Application Laid-Open Publication No. 62-132099, a so-called xe2x80x9cwide anglexe2x80x9d mast apparatus allowing for an operator to have a wide front sight.
FIG. 1 is a schematic plan of a conventional wide sight mast apparatus.
The conventional mast apparatus has as mast members thereof a pair of left and right inner mast members 105, a pair of left and right middle mast members 106, and a pair of left and right outer mast members 102, and further includes as hydraulic cylinders associated therewith a pair of left and right lift cylinders 101 disposed behind the outer mast members 102, and a pair of left and right free lift cylinders 104 arranged behind the inner mast members 105, for lifting a pair of left and right lift brackets 103 fixed to a finger bar 107.
The conventional mast apparatus is different from a mast apparatus having a free lift cylinder of a relatively large diameter between left and right mast members (see FIG. 12B), in that an operator on a driver""s seat has a front sight free of an interruption at a central region of the sight. However, behind the left and right mast members 102, 105 and 106, there are installed mast drives, such as a chain and a hydraulic piping, interrupting oblique front sights of the operator, splitting them, killing in part, needing extra considerations, such as for a coping with a delicate shift of the eye point, such as when visually checking a fork action.
A conventional mast apparatus of a triple mast type is disclosed in Japanese Utility Model Application Laid-Open Publication No. 59-123096.
This conventional mast apparatus includes an inner mast member with which a lift bracket is supported and which has a free lift cylinder mounted thereon for lifting the lift bracket, a middle mast member, and an outer mast member which is provided with a lift cylinder for elevating the middle mast member. The lift bracket is elevated by operating the free lift cylinder until it reaches an upper limit relative to the inner mast member, before an operation of the lift cylinder by which the middle mast member is elevated at a speed and with which the inner mast member is caused to ascend by a double stroke at a double speed, as it is driven with a chain in dependence on an elevation of the middle mast member.
In this mast apparatus, the inner, middle and outer mast members are arranged so that they have an even height when the lift bracket is positioned at a lower limit, with a resultant restriction to a free lift stroke of the lift bracket that is relatively short.
The present invention has been achieved with such points in view.
It therefore is an object of the invention to provide a mast apparatus having a mast drive possibly kept from interrupting an operator""s sight, as well as from splitting the sight or killing the same in part, thereby permitting a relatively wide defined sight.
Another object of the invention is to provide a mast apparatus of a triple mast type with a relatively long free lift stroke.
An aspect of the invention provides a mast apparatus of a triple mast type as a slide type full free lift mast, the mast apparatus comprising a mast combination composed of an inner mast member, a middle mast member and an outer mast member, a combination of a free lift cylinder and a lift cylinder provided for the mast combination, and a chain for driving the inner mast member, the chain intervening between the inner mast member and the middle mast member.
According to this aspect of the invention, a drive chain for an inner mast member intervenes between the inner mast member and a middle mast member, which either or both overlap the chain in a sight angle of an operator. In other words, xe2x80x9ca region of sight field to be killed by the chainxe2x80x9d (hereafter referred to as xe2x80x9ca first dead zonexe2x80x9d) overlaps xe2x80x9ca region of sight field killed by the inner mast member and/or the middle mast memberxe2x80x9d (hereafter referred to as xe2x80x9ca second dead zonexe2x80x9d), and an interruption of the chain that would have been visible in a sight of the operator is eliminated in dependence on a degree of overlap between the first dead zone and the second dead zone, with a reduced frequency of occurrences of sight split and a reduced degree of sight killing, allowing for the wider defined sight.
Further, a mast apparatus according to the aspect of the invention may have a chain drive including a chain wheel and a chain applied over the chain wheel. Therefore, according to the invention, an intervening chain between inner and outer mast members may be adaptive for a transverse chain drive arrangement, in which a chain wheel may have a transversely extending wheel face that may effectively overlap a combination of mast members, when observed in a longitudinal direction of a fork lift truck, permitting an outside dimension of the mast apparatus to be reduced in the longitudinal direction, allowing for the fork lift truck to have a relatively short front overhang between a front axle and the lift bracket.
Another aspect of the invention provides a mast apparatus of a triple mast type as a full free lift mast provided with a lift bracket, the mast apparatus comprising a mast combination composed of an inner mast member, a middle mast member and an outer mast member, the inner mast member being higher in top level than the middle and outer mast members, as the lift bracket is held in a lowest position thereof, a chain for driving the inner mast member to be elevated and lowered, and a chain wheel for the chain to be applied thereover, the chain wheel being provided at an upper end part of the middle mast member and residing between a top level of the inner mast member and a top level of the middle mast member.
According to this aspect of the invention, a mast apparatus of a triple mast type as a full free lift mast has an inner mast member higher in a set-up top level (in terms of a lowered mast height) than middle and outer mast members, and an inner-mast-lifting chain wheel provided at an upper end part of the middle mast member and accommodated within a difference between top levels of the inner and middle mast members, so that a free lift stroke for a lift bracket to be solely elevated and lowered relative to the inner mast member depends on a length of the inner mast member, without restriction from a length of the middle or outer mast member, and is allowed to have a long dimension, in addition to that a chain wheel projecting over the middle mast member is kept from exceeding the top level of the inner mast member, and is protected by the inner mast member against a collision with external obstacles, such as when a fork lift truck equipped with the mast apparatus travels.
In other words, as an inner mast member is taller than a middle mast member by a predetermined difference of height, as well as than an outer mast member, the free lift stroke for a lift bracket to be moved relative to the inner mast member depends on a length of this mast member, in free of restrictions due to a height of the middle or outer mast member.
Further, a chain wheel for an inner-mast-driving chain is accommodated within a height difference between from the middle mast member to the inner mast members, i.e., its projection height over that mast member never exceeds a height of this mast member, which mast height effectively protects the chain wheel against collisions that otherwise might occur, such as with an external structure.